270 MORPHOLOGY AND EVOLUTION
their condition precedes the menstrual discharge. After making a careful series of records, and noting how menstruation ceased during an attack of influenza, Pocock concludes , . . . <l it may be laid down as an established fact that in Gibbons the interval between the menstrual discharges is a little over the calendar month, and that the discharge continues for from two to three days." In the Chimpanzee menstruation begins at the tenth year. It begins every twenty-eight days, and the discharge, which flows for three days, amounts to four or six ounces. Six days before each period the animal is in heat, the labia become swollen and turgid, and the nipples become erect and full. After the discharge appears the swelling of the external genital organs subsides. Keith points out that seasonal changes have no modifying influence on the periodicity of the menstrual flow. Nothing is known of the duration, periodicity or quantity of the menstrual discharge in the Gorilla and Orang. -The genitalia become turgid and highly coloured before menstruation in the Cercopithecidse, so the Chimpanzee resembles them, whereas the Gibbon does not.
The ovum in the Apes has been described by Hubrecht and its characters are generally similar to those in Man.
The placenta consists of a single disc.
Section C.—THE MALE GENERATIVE ORGANS.
It has been stated by several writers that the external generative organs are small, but that is not the case in the male Chimpanzee. The penis in that Ape is long and tapering. No pronounced glans is present, but the lips of the vertical meatus urinarius are thickened.